Enameled metal substrates and method of forming

ABSTRACT

Pinhole-free enamel coatings have a dielectric breakdown above 2,500 volts are formed atop a multiperforated metallic substrate by the addition of an inert refractory additive, e.g., silicon dioxide, magnesium oxide, calcium fluoride, etc., to the enamel slurry utilized to coat the substrate. The inert additives comprise between 10 and 35 percent by weight of the enamel frit in the slurry and remain physically unaltered during subsequent firing of the substrate to increase the viscosity of the fired enamel thereby assuring an edge coverage of at least 5 mils for enamel coatings less than 10 mils thick.

United States Patent Girard [54] ENAMELED METAL SUBSTRATES AND METHOD OF FORMING [72] Inventor: Roland T. Girard, Scotia, N.Y.

[73] Assignee: General Electric Company [22] Filed: Dec. 15, 1969 1211 Appl. No.: 885,206

521 U.S.Cl ..l17/222,ll7/98,li7/l29, 117/230, 204/181 511 1111.131. ..C23d5/02 [58] FieldoiSearch ..1l7/222,l29,2l0,98; 204/181 [56] References Cited UNITED STATES PATENTS 2,175,689 10/1939 Gallup ..117/129x 3,338,732 8/1967 Holcomb ..l l7/129 X Feb. l,1972

3,476,584 11/1969 Randklev et al. ..1 17/129 X Primary Examiner-William L. Jarvis Attorney-Richard R. Brainard, Paul A. Frank, John J. Kissane, Frank L. Neuhauser, Oscar B. Waddell and Joseph B Forman 1 1 ABSTRACT Pinhole-free enamel coatings have a dielectric breakdown above 2,500 volts are formed atop a 'multiperforatcd metallic substrate by the addition of an inert refractory additive, e.g., silicon dioxide, magnesium oxide, calcium fluoride, etc., to the enamel slurry utilized to coat the substrate. The inert additives comprise between 10 and 35 percent by weight of the enamel frit in the slurry and remain physically unaltered during subsequent firing of the substrate to increase the viscosity of the fired enamel thereby assuring an edge coverage of at least 5 mils for enamel coatings less than l0 mils thick.

8 Claims, 5 Drawing Figures mmanm. 1 rare $639,164

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ENAMELEID METAL SUBSTRATES AND METHOD OF FORMING ENAMELED METAL SUBSTRATES AND METHOD OF FORMING This invention relates to vitreous enamel-coated multiapertured metallic sheets and to a method of forming such sheets. In a more particular aspect, the invention relates to a technique for providing superior edge coverage at apertures in the metallic sheet by the addition of a refractory additive to the vitreous enamel aqueous slurry employed to coat the sheet to reduce surface tension during subsequent firing of the enamel.

Vitreous enamel coatings characteristically are formed by immersing a metallic substrate in an aqueous slurry of ground enamel frit and subsequently firing the coated substrate at a temperature in excess of 600 C. to mature the enamel. When the substrate is multiapertured, e.g., each substrate for a radio receiver customarily has in excess of 100 apertures of various dimensions passing therethrough, surface tension both in the liquid slurry and in the softened enamel during firing tends to reduce the edge coverage at the apertures inhibiting utilization of the enamel-coated substrate as an electrically insulated ground plane. While spray coating of an apertured substrate produces slightly less surface tension in the liquid slurry due to evaporation of water during spraying, surface tension in the enamel during firing is not substantially affected by spray application of the vitreous enamel. Moreover, the inability of spray coating to adequately cover the sidewalls of small diameter apertures e.g., apertures less than one-sixteenth inch diameter, generally negates the independent utilization of spray techniques for coating multiaperatured substrates. Even the superior substrate coverage afforded by electrophoretic coating techniques is not sufficient to inhibit inadequate aperture edge coverage resulting from surface tension during firing of the enamel.

l have discovered that 3- to -mil thick vitreous enamel electrically insulating coatings can be formed atop a mu]- tiapertured metallic substrate by the addition to the aqueous enamel slurry of a refractory additive in quantities between 10 and 35 percent by weight of the vitreous enamel frit therein. The refractory additives are substantially nonreactive with the enamel and possess a melting temperature above the firing temperature of the enamel to provide stable adhesion sites increasing the viscosity of the enamel during subsequent firing.

While fillers such as feldspar and quartz have been added to enamel coatings to minimize sagging during firing, these additives heretofore have been limited to quantities below 7 percent by weight of the enamel frit content. It also has been suggested (in copending application Ser. No. 824,948, filed May 15, 1969 in the name of R. T. Girard and M. J. Curran and assigned to the assignee of the present invention) that relatively inert fillers selected from the group consisting of magnesium oxide, silicon dioxide, lithium titanate and mixtures thereof be added in quantities between 20 and 30 percent by volume to the enamel particles forming a fluidized bed to reduce surface tension in the enamel during subsequent firing of the coated substrate. The incorporation of refractory particles between 325 and +400 mesh in an aqueous slurry of vitreous enamel heretofore has not been proposed.

It is therefore an object of this invention to provide a method of aqueously depositing pinhole free enamel coatings atop an apertured conductive substrate.

It is also an object of this invention to provide a method of forming pinhole-free vitreous enamel coatings atop an apertured substrate utilizing low-temperature deposition techniques.

It is a further object of this invention to provide a method of forming electrically-insulating enamel coatings atop a perforated substrate by electrophoretic deposition techniques.

It is a still further object of this invention to provide a novel enamel-coated apertured substrate suitable for deposition of electronic components thereon.

These and other objects of this invention are achieved by the addition to an aqueous slurry containing essentially 15-60 percent by weight vitreous enamel frit suspended in water of refractory particles in quantities between 10 and 35 percent by weight of the enamel frit. The refractory particles are of a dimension between 325 and +400 mesh and possess a softening temperature in excess of the firing temperature of the enamel while being substantially nonreactive with the enamel frit at the firing temperature. Upon subsequent firing of the coated substrate at temperatures above 600 C., the refractory particles remain substantially unchanged to increase the viscosity of the fired enamel and provide superior edge coverage. Thus, an enameled coated substrate suitable for the deposition of the electronic components thereon charac teristically includes a multiapertured metallic sheet less than 35 mils thick having an insulating coating thereon consisting essentially of vitreous enamel and 10 to 40 percent by weight inert refractory oxide particles of a dimension between 325 and +400 mesh.

The novel features believed characteristics of the invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof may best be understood by reference to the following description, taken in connection with the accompanying drawings, in which:

FIG. 1 is a partially broken away enlarged isometric view of a perforated substrate to be enamel coated in accordance with this invention;

FIG. 2 is a flow chart illustrating in block diagram form the method of this invention utilizing clip coating techniques;

FIG. 3 is a partially broken away enlarged isometric view of an enamel coated substrate in accordance with this invention;

FIG. 4 is a flow chart illustrating in block diagram form the preferred method of electrophoretically depositing a pinhole free enamel coating atop an apertured substrate, and

FIG. 5 is a sectional view of an electrophoretic bath suitable for enameling a substrate in accordance with the techniques of FIG. 4.

The substrate to be coated in accordance with this invention is illustrated in FIG. 1 and generally comprises a metallic sheet 10 of, for example, enameling iron, i.e., iron containing less than 0.2 percent carbon, having a plurality of apertures 12 extending completely therethrough. Typically, sheet 10 is less than 35 mils thick and contains in excess of rectangular and/or circular apertures with the width of the apertures varying between approximately one-sixteenth and one-fourth inch, dependent upon the desired usage for the aperture, e.g., whether the apertures are to serve for mounting components or for the passage therethrough of electrical wiring. Desirably, the edges of the apertures are rounded at a radius approximately one-half the thickness of sheet 10 to minimize surface tension effects during subsequent firing of the enamel.

To dip-coat sheet 10 in accordance with this invention, as illustrated by the flow chart of FIG. 2, the sheet is prepared for enameling by conventional sand blasting or pickling techniques and immersed in an aqueous slurry of vitreous enamel containing inert refractory additives in quantities between 10 and 35 percent by weight of the enamel frit in the slurry to impart resistance to enamel flow during subsequent firing of the coated sheet. Generally, the vitreous enamel employed in the slurry contains less than 60 percent by weight sodium oxide, potassium oxide or mixtures thereof to assure high electrical resistivity in the deposited enamel coating.

Because the refractory additives serve only to increase viscosity during firing by providing particulate adhesion sites for the softened enamel, any particulate material, such as silicon dioxide, magnesium oxide, calcium fluoride, nephalite, lithium titanate, aluminum oxide, etc., having a softening temperature in excess of the enamel while being relatively nonreactive with the enamel at the firing temperature of the enamel can be employed in the practice of this invention. Oxides, such as potassium oxide and sodium oxide, reactive with the vitreous enamel by fluxing the enamel to produce a lower melting point should be avoided together with refractory oxides deleterious to the electrical characteristics of the enamel. In general, the refractory additives utilized in this invention should be characterized by a softening temperature above 1,000 C. and should exhibit an electrical resistivity at least equal to the electrical resistivity of the vitreous enamel utilized for the coating. Prior to being added to the slurry, the refractory additives desirably are ground to a dimension between 325 and +400 mesh to assure proper suspension of the particles within the slurry and adequate dispersion of the refractory additives along the surface of sheet 10 while being of sufficient diameter to produce a viscosity in the vitreous enamel coating inhibiting flow of the enamel during firing.

The refractory additives also advantageously should be characterized by a temperature coefficient of expansion between 9X10 and 1 1X10 cm./cm./ C., i.e., within 2X10 cm./cm./ C. of the sheet to be coated, to minimize thermally induced stress in the finished enamel coating while assuring a compressive stress therein during operating conditions when heat generated within electronic components mounted upon the enamel coated sheet is conducted to the undeflyiiig sheet. Among the inert refractory additives exhibiting the desired high thermal expansion characteristics are silicon dioxide. magnesium dioxide, calcium fluoride, nephatite, lithium titanate, and mixtures of the foregoing additives, and the chosen refractory additive is added to the vitreous enamel slurry in concentrations between 10 and 35 percent by weight of the enamel fn't in the slurry. The good electrical insulating sing si waq ea fle and. magnesium.- oxide in assobiat ion with the high thermal expansion coefficients exhibited by these materials make these refractory oxides highly preferred in the practice of this invention. In general, superior edge coverage is obtained when the additives form between 25 percent and 35 percent by weight of the enamel frit in the slurry.

When a refractory additive having a thermal coefficient of expansion in excess of 2X10 cm./cm./ C. below the thermal coefficient of expansion of the underlying metallic sheet is employed in the enameled slurry, e.g., when aluminum oxide having a thermal coefficient of expansion between 7X10 and 8X 10cm./cm./ C. is an additive for enamel coating atop an iron substrate having athermal coefficient of expansion between 1 l and I and 13x10 cm./cm./ C. the refractory additive should be present in concentrations not higher than concentrations between [0 percent and 15 percent by weight of the enamel are preferred when the refractory additive possesses a thermal coefficient of expansion more than 2X10 cm./cm./ C. below the thermal coefficient of expansion of the metalsheet.

The remainder of the enamel slurry employed for coating sheet is generally conventional in composition and basically includes an aqueous suspension of enamel particles, e. g., commercial ground coat enamel sold by Ferro Corporation as No. 2232 enamel, wherein the enamel particles form between and 60 percent by weight of the slurry dependent upon the viscosity desired and the type of enamel application employed. For example, a high viscosity, e.g., an enamel frit to water weight ratio above 80 percent is desirable for dip coating while a lower viscosity of approximately 30 percent is desirable for electrophoretic deposition. For dip coating enamel applications, an electrolytic salt such as sodium nitrite,

"potassium carbonate, sodium aluminate, magnesium carbonate, borax or mixtures of the foregoing, conventionally is added to the slurry in quantities below 1 percent by weight of the enamel particles to produce a thixotropy tending to enhance adhesion of the enamel slurry to the sheet. Similarly, a suspending agent, such as clay, or bentonite, is normally present in the dip coating slurry in quantities between 4 and 7 percent to assist in obtaining the desired thixotropy in the slurry and in suspending both the enamel and inert particles therein. Organic materials, e.g., starch or a gum such as gum tragacarth or gum karaya also can be used to assist in binding and suspending the solids in the slurry. The amount of suspending agent can be lowered considerably, however, when the slurry is agitated prior to immersion of the sheet therein and when the sheet is transported from the bath to the firing chamber in a plane parallel to the direction of travel to inhibit runoff of the vitreous enamel from the sheet.

A conventional slurry suitable for dip coating sheet 10 specifically may comprise an aqueous solution of approxi mately 100 parts by weight ground coat enamel, e.g., No. 2232 enamel sold by Ferro Corp suspended in parts by weight water with 6 parts by weight clay being added to provide enhanced suspending characteristics and thixotropy in the slurry. Minor amounts of an electrolyte, for example, a mixture of A: parts by weight sodium nitride, /s parts by weight magnesium carbonate and 1/16 parts by weight sodium aluminate, also desirably are present in the slurry to enhance the rheological properties of the slurry, In accordance with the practice of this invention, 30 parts by weight of an inert refractory additive, such as silica, ground to a dimension of -200 mesh is added to the slurry and the slurry is brought to a specific gravity of approximately 1.6 for dip coating. Metallic sheet 10 is then dipped one or more times into the slurry to completely coat the sheet with an enamel layer approximately 12 mils thick whereupon the coated sheet is dried and fired at a temperature of approximately 830 C. for 3 /2 minutes, e.g., utilizing the RF oven illustrated in prior-mentioned copending U.S. Pat. application Ser. No. 824,948 (specially incorporated herein by reference), to mature the enamel layer to a pinholefree coating between 6 and 7 mils thick. During firing, the refractory particles remain stable-providing adhesion sites for the porcelain enamel thereby reducing the effect of surface tension tending to draw the enamel from the edges of the apertures in sheet 10. In general, the matured enamel layer is less than lO-mils thick (with thicknesses between 6 and 7 mils being typical) while providing at least 5 mils coverage at the edges of the apertures. Y

While the disclosure of FIG. 2 is specifically directed to dipcoating techniques, sheet 10 also can be coated with an enamel inert particle aqueous solution utilizing conventional flow coating techniques. Similarly, although spray coating does not provide sufficient coverage of the sidewalls of small diameter apertures, e.g., apertures below one-sixteenth inch diameter as are present in sheets contemplated for utilization as radio chassis, spray-coating techniques advantageously can be employed to build up the enamel layer subsequent to an initial coating of the apertured sidewalls utilizing either flow or dip-coatingtechniqu e s. H l v D v After firing of the slurry-coated sheet 10, the structure of FIG. 3 is formed characterized by a metallic sheet 10 less than 35 mils in thickness and having at least apertures therein underlying an enamel coating 14 less than 10 mils thick. The enamel coating itself generally is comprised of approximately 65 to 75 percent fused enamel having dispersed therein between 25 and 35 percent by weight inert refractory oxide particles of a dimension between 325 and +400 mesh. The vitreous enamel coated sheet exhibits a dielectric breakdown strength above 2,500 volts and suitably serves to accept thick and thin film deposition of electronic components thereon. For example, a capacitor 15 may be formed by deposition of a palladium silver electrode 16 thereon with electrically grounded sheet 10 forming the counterelectrode while cermet resistors and copper interconnections identified by reference numerals l7 and 18, respectively, can be vacuum deposited atop the enamel coating. Those components, e.g., a tunable condenser 19, not amenable to vacuum or thick film deposition are mechanically mounted to the coated sheet by fastening means such as bolts 20 extending through apertures in the substrate p The inert refractory additives also can be employed advantageously during coating of sheet 10 by electrophoretic techniques as exemplified by the flow chart of FIG. 4, i.e., by

electrophoretically depositing an enamel frit and inert additive admixture atop the surface of multiperforated sheet and subsequently utilizing the inert additives as enamel adhesion sites during subsequent firing of the enamel to reduce surface tension in the fired enamel. The electrophoretic bath 22 (illustrated in FIG. 5 and discussed more fully in my application entitled Method of Electrophoretically Depositing Pinhole Free Enamel Coatings" filed concurrently herewith) for deposition of the enamel coating primarily is formed of an aqueous suspension of between to 60 percent by weight enamel frit in water with a relatively low viscosity bath having an enamel/water weight ratio between 15 and 40 percent being preferred for electrophoretic deposition. The bath also contains an anionic colloidal material, e.g., preferably ammonium alginate, in quantities below 3 percent by weight of water in the bath to plate out simultaneously with the enamel atop sheet 10 thereby sealing the bath from the sheet to inhibit electrolysis while permitting continued plating of the enamel upon the sheet. To further assist in sealing the plated electrode while inhibiting excessive buildup of the enamel at the edges of the substrate, a commercially available acrylic emulsion, e.g., acrylic emulsion E269 sold by Rohm and Haas, is added to the bath in quantities less than 0.25 percent by weight of the enamel frit. In general, the acrylic emulsion should be present in approximately 65 to 85 percent by weight of the chosen anionic colloidal material to inhibit frothing due to the generation of gases during plating while permitting subsequent firing without excessive residue in the enamel. Minor quantities of other items, for example, commercially available wetting agents, such as Tergitol, also can be added to the bath in quantities less than one-half percent by weight of water, if desired.

In a particularly preferred electrophoretic bath, an aqueous enamel bath containing 30 parts by weight enamel of a particle dimension between +400 and 325 mesh is mixed with 100 parts by weight water and a small amount, e.g., 0.1 part by weight of water, of both ammonium alginate and acrylic emulsion E269 are added to the solution to prevent frothing at the anode. An anionic wetting agent such as Tergitol suitably is added to the bath in very small quantities, e.g., 1/20 part by weight, along with 30 parts by weight silicon dioxide ground to a diameter of 200 mesh. After pickling apertured sheet 10 utilizing conventional techniques, the sheet is immersed within the electrophoretic bath intermediate inactive cathode electrodes 24 and a potential of approximately 30 volts is applied to the bath from source 26 to produce a current flow of 2.5 amps at sheet 10. Plating is continued until a coating between 12 and 13 mils thick is deposited atop the sheet whereupon the sheet is removed from the bath, dried, and placed within a furnace at a temperature of 830 C. for 3% minutes to mature the enamel to a vitreous enamel layer between 6 and 7 mils thick atop the apertured sheet. Because of the presence of silicon dioxide particles in the enamel, an edge coverage in excess of 5 mils is achieved at apertures 12 passing through the substrate.

What I claim as new and desire to secure by Letters Patent of the United States is: I

l. in a method of forming an enamel coating atop a metallic substrate by immersing said substrate in an aqueous slurry containing essentially 15-60 percent by weight vitreous enamel suspended therein and subsequently firing the coated substrate at temperatures above 600 'C. to seal said enamel coating atop said substrate, the improvement comprising the addition of between 325 to +400 mesh refractory additive particles to the slurry in quantities between 10 and 40 percent by weight of the enamel frit for the coating of multiperforate substrates, said refractory oxide additive being substantially nonreactive with said enamel frit at the enamel firing temperature and having a softening temperature in excess of the firing temperature of said enamel to increase the viscosity of said fire enamel and provide superior coverage at the edges of perforations extending through the substrate.

2. A method of forming an enamel coating atop a metallic substrate according to claim 1 wherein said refractory additive is a material selected from the group consisting of SlllCOl'l dioxide, magnesium oxide, calcium fluoride, aluminum oxide, nephalite, lithium titanate and mixtures of the foregoing additives with another additive of the group.

3. A method of forming an enamel coating atop a metallic substrate according to claim 2 wherein said refractory additive is selected from the group consisting of silicon dioxide, magnesium oxide, calcium fluoride, nephalite, lithium titanate, and mixtures of the foregoing additives with other additives of the group, said inert additive being present in quantities between 25 and 35 percent by weight of the enamel frit.

4. A method of forming an enamel coating atop a metallic substrate according to claim 3 wherein said fired enamel coating is less than 10 mils thick.

5. A method of forming an enamel coating atop a metallic substrate according to claim 2 wherein said inert additive is aluminum oxide in quantities between l0 and 15 percent by weight of the enamel frit.

6. An electrically insulated substrate suitable for deposition of electrical components thereon comprising a multiapertured metallic sheet less than 35 mils thick having an insulating coating thereon less than 10 mils thick, said coating consisting of vitreous enamel and l0 to 40 percent by weight refractory particles of a dimension between 325 and +400 mesh, said refractory particles being of a material substantially nonreactive with said enamel frit at the enamel firing temperature and having a softening temperature in excess of the firing temperature of said enamel.

7. An electrically insulated substrate according to claim 6 wherein said refractory particles are selected from the group consisting of silicon dioxide, magnesium oxide, calcium fluoride, aluminum oxide, nephalite, lithium titanate, and mixtures thereof of the foregoing additives with another additive of the group.

8. An electrically insulated substrate according to claim 7 wherein said refractory particles are silicon dioxide in quantities between 25 and 35 percent by weight of the frit. 

2. A method of forming an enamel coating atop a metallic substrate according to claim 1 wherein said refractory additive is a material selected from the group consisting of silicon dioxide, magnesium oxide, calcium fluoride, aluminum oxide, nephalite, lithium titanate and mixtures of the foregoing additives with another additive of the group.
 3. A method of forming an enamel coating atop a metallic substrate according to claim 2 wherein said Refractory additive is selected from the group consisting of silicon dioxide, magnesium oxide, calcium fluoride, nephalite, lithium titanate, and mixtures of the foregoing additives with other additives of the group, said inert additive being present in quantities between 25 and 35 percent by weight of the enamel frit.
 4. A method of forming an enamel coating atop a metallic substrate according to claim 3 wherein said fired enamel coating is less than 10 mils thick.
 5. A method of forming an enamel coating atop a metallic substrate according to claim 2 wherein said inert additive is aluminum oxide in quantities between 10 and 15 percent by weight of the enamel frit.
 6. An electrically insulated substrate suitable for deposition of electrical components thereon comprising a multiapertured metallic sheet less than 35 mils thick having an insulating coating thereon less than 10 mils thick, said coating consisting of vitreous enamel and 10 to 40 percent by weight refractory particles of a dimension between -325 and +400 mesh, said refractory particles being of a material substantially nonreactive with said enamel frit at the enamel firing temperature and having a softening temperature in excess of the firing temperature of said enamel.
 7. An electrically insulated substrate according to claim 6 wherein said refractory particles are selected from the group consisting of silicon dioxide, magnesium oxide, calcium fluoride, aluminum oxide, nephalite, lithium titanate, and mixtures thereof of the foregoing additives with another additive of the group.
 8. An electrically insulated substrate according to claim 7 wherein said refractory particles are silicon dioxide in quantities between 25 and 35 percent by weight of the frit. 